In recent years, there has been growing concern to produce energy-efficient lubricated components. Moreover, modern engine oil specifications require lubricants to demonstrate fuel efficiency in standardized engine tests. The thickness and frictional characteristics of lubricant films are known to affect the fuel economy properties of oils.
When rubbing surfaces in a machine (engine, gear system or transmission) come into contact, a frictional force exists that retards the motion of the surfaces. This frictional force, called boundary friction, reduces the efficiency of the machine. Boundary friction coefficients may be measured for a lubricant composition using the high frequency reciprocating rig (HFRR). The boundary friction measured in the HFRR is known to be related to fuel efficiency in vehicles. The ability of the lubricant composition to reduce boundary layer friction is reflected by the determined boundary lubrication regime coefficient of friction (COF). A lower value is indicative of lower friction and thus improved fuel economy.
The present disclosure relates to a lubricant additive, method for reducing a boundary friction coefficient of a lubricant composition, and method for improving fuel economy. The additive includes a synergistic mixture of a) a metal-containing phosphorus antiwear compound derived from at least one secondary alcohol in an amount sufficient to provide the lubricant composition with from about 200 to about 1000 ppm by weight phosphorus, and b) a polyol derived from a diol and a mono-ol having a diol to mono-ol molar ratio ranging from about 0.3:1 to about 2.0:1, wherein the diol contains from 6 to 36 carbon atoms and the mono-ol contains from 12 to 16 carbon atoms. The polyol is present in the lubricant additive in an amount sufficient to provide a synergistic reduction in the boundary friction coefficient of the lubricant composition in combination with component (a).
Another embodiment of the disclosure provides a method for synergistically reducing a boundary friction coefficient of a lubricant composition. The method includes combining a base oil of lubricating viscosity having a first boundary friction coefficient with a lubricant additive containing a) a metal-containing phosphorus antiwear compound derived from at least one secondary alcohol in an amount sufficient to provide the lubricant composition with from about 200 to about 1000 ppm by weight phosphorus, and b) a polyol derived from a diol and a mono-ol having a diol to mono-ol molar ratio ranging from about 0.3:1 to about 2.0:1, wherein the diol contains from 6 to 36 carbon atoms and the mono-ol contains from 12 to 16 carbon atoms. The polyol is present in the lubricant additive in combination with component (a) in an amount sufficient to provide a second boundary friction coefficient of the lubricant composition that is less than the first boundary friction coefficient of the lubricant composition.
Yet another embodiment of the disclosure provides a method for improving the fuel economy of a vehicle. The method includes lubricating the vehicle with a lubricant composition that includes a) a base oil of lubricating viscosity; b) a metal-containing phosphorus antiwear compound derived from at least one secondary alcohol in an amount sufficient to provide the lubricant composition with from about 200 to about 1000 ppm by weight phosphorus; and c) a polyol derived from a diol and a mono-ol having a diol to mono-ol molar ratio ranging from about 0.3:1 to about 2.0:1, wherein the diol contains from 6 to 36 carbon atoms and the mono-ol contains from 12 to 16 carbon atoms. The polyol is present in the lubricant composition in combination with component (b) in an amount sufficient to provide a boundary friction coefficient of the lubricant composition that is synergistically less than a boundary friction coefficient of the lubricant composition containing only one of component (b) or component (c).
An unexpected advantage of the additive and methods described herein is that the boundary coefficient of friction is reduced by the combination of metal-containing phosphorus antiwear compound and polyol despite the fact that the same polyol may actually increase the boundary friction coefficient of the base oil in the absence of the metal-containing phosphorus antiwear compound. Additional the boundary coefficient of friction may also be lower than the boundary coefficient of friction provided by the metal-containing phosphorus antiwear compound in the absence of the polyol.